Hidden among the many floors of Hogwarts is none other than the famous Room of Requirement, the very same room Dumbledore's Army trained in; the same room Rowena Ravenclaw's diadem had been hidden away before the Second Wizarding War. To enter the Room of Requirement, one must walk past the doors three times with a clear desire in mind.  However, the room may not look exactly as you expect it to when you encounter it.

The room has no one appearance; it shifts and changes based on the user's desires. Whatever you needed in that moment, whether it be a quiet place to read or a running a covert operations scheme, it would change its interior and supply whatever you needed. The room cannot, however, endlessly produce food or money as that is against Gamp's Law. People didn't expect it to still function after having been ravaged by Fiendfyre in the war, but it is not so easy to destroy ancient magic.

That said, the room is not entirely hidden from the rest of the world. One can still enter another's version of the Room of Requirement as long as they knew and had the desire to enter it. If you really want to keep people out, you will have to be pretty specific.